Such a heat exchanger conventionally comprises an inlet fluid manifold and an outlet fluid manifold, the manifolds being interconnected by at least one group of tubes extending in zigzag manner in a vertical plane between the inlet manifold and the outlet manifold. The zigzags define hairpins having adjacent branches which slope relative to one another (at an angle approximately in the range 8.degree. to 15.degree.) while leaving empty space between them. The slope is necessary to enable bubbles of fluid vapor to flow naturally in the tubes.
High-power installations require relatively large heat exchange areas. Obtaining such areas involves providing a greater number of heat exchangers or providing a heat exchanger that is large in size and whose design is dictated by the cross flow of the solids.